Please note, this is a rewrite.
I felt need to do so, mainly because of OTaylor's (completely reasonable) complaint regarding the magnitude of the forces involved. A point which I managed to bury so deeply in the post, that its true emphasis is likely to have been missed.
I should have led with a magnitude argument, placing the arguments regarding 'hold' or 'suction' phenomena versus all the other forces acting on the rail (and all the other wetted surfaces) in perspective. In a nutshell, there are a lot of forces acting on the wetted rails, particularly on the waveside wetted rails, all of which have substantial magnitude. And aside from the surfer's action via his posture etc. to counterbalance them they are all acting to 'displace the rail' and rotate the board out of the wave.
In the “Hydrofoils and Lift” thread, lalabrooklyn described a curious effect which he reasonably attributed to his rear rails ( see thread for his description and a picture of his board ). Also, surffoils recently started a thread “Design misconceptions and urban myths” in which he directs the reader to another site which offers an interesting discussion of design elements etc., among which is the role of rail profile. (By the way, there's also a boatload of similar discussions to be found in the Swaylocks Archives – they're just not as organized.)
As the surfer moves transversely across the wave face his wave side rail is interacting with the flow in the wave face, which is both upward and forwards. But its also interacting with a transverse flow, that is the flow parallel to the rail by virtue of the boards transverse motion across the face. This transverse motion will, in a manner similar to planing tend to displace the rail from the face -i.e. pushing it out of the face.
However, at the same time the wave is flowing forward, and this flow will try to also carry the rail in that direction. But the rail (obviously connected to the rest of the board and effectively the surfer too) has inertia and must be accelerated. But there will be a lag in its ability to do so, hence the rail will appear to sink into the wave. This is basically the same effect that I described in my initial post.
The degree to which it sinks is not only a function of inertia, but also of the extent of the surface presented to the flow. A larger surface for a given inertial (mass or weight of surfboard and surfer) the less it will appear to sink into the wave. Razor thin rails appearing to sink more that fatter rails.
What about the differential forward flow? This, and if there is any 'holding' or 'sucking' phenomena involved, are minor in comparison to the above described forces and effects. And to go a step further, I'm inclined to place 'holding' and 'sucking' at the bottom of the list in terms of magnitude.
But I have to admit that in the end, at present I have absolutely no proof (in hand, at least at the moment) as to which of these effects is dominating. But the stickiness of fluids and holding effects tend to have upper limits in terms of force, whereas a purely mechanic effect as suggested by this consequences of the transverse, forward, and even upward flows really don't, at least their range can be pretty large.
The differential forward flow also has implications in contributing to the torques that the board is subjected to. A given rail and bottom leading itself to being rolled out of the face, more so than another.
Back to my rewrite apology... there no excuse for initially screwing this post up. My apologies.
kc
